The present invention relates to the field of aeroengines, e.g. turbojets, and it relates more particularly to fastening outlet guide vanes (OGVs) between an intermediate casing hub and an intermediate casing outer shroud of such an engine downstream from its fan, the assembly forming that which may be referred to as an intermediate casing OGV wheel.
In such engines, where weight saving is desired, proposals have been made for guide vanes to be made out of polymer matrix composite material reinforced by fibers, e.g. woven in three dimensions (3D woven preforms), while the other components of the wheel, i.e. its hub and the outer shroud, are generally made of metal.
As shown in FIG. 6, the intermediate casing OGV wheel essentially comprises a hub 10 having upstream and downstream flanges 12 and 14 connected together by longitudinal connection arms 16, together with the outer shroud 18. The composite guide vanes 20, which may be thought of as being I-shaped, may be bolted directly firstly to the flanges 12 and 14 of the intermediate casing and secondly to the outer shroud 18 of the intermediate casing. These guide vanes may be structural in the sense that forces are transmitted via the vanes between the outer shroud and the hub. Such an arrangement for an OGV wheel can then present numerous drawbacks:
Firstly, drilling leads to fibers in the preform being broken, thereby leading to local weakening. When heavily stressed, such a zone often gives rise to crack starters, which cracks then propagate often sufficiently for the parts to break.
Secondly, the small contact area under the washer and the screw head, taken together with a high level of tightening as is necessary for fastening purposes, gives rise to very high flattening pressures. The strongly prestressed material then becomes weakened as from the assembly sequence, and when the coefficient of friction is no longer sufficient to withstand loading, the composite part comes into abutment against the shank of the screw. It is then loaded essentially in shear, which is a mode of deformation that composite material withstands poorly. The potential damage may also be made worse by the very small area of contact between the shank of the screw and the portion of the OGV coming into contact therewith.
Furthermore, the shear moduluses of composite materials, which are small in comparison with those of metal materials, do not make it possible to guarantee that stresses are spread spatially sufficiently beyond the fastening zones. The distribution of stresses within the part is therefore very non-uniform in such zones since forces pass mainly along very narrow “strips” lying between two points of fastening. Stresses are therefore at an excessive level in such “strips” and very small elsewhere (a very steep stress gradient), which has a direct impact on the mechanical strength of the part (high stress) and on its overall stiffness (a large amount of deformation between fastenings).
Finally, the rigid embedding does not make it possible to distinguish between directions. Specifically, the forces passing through the guide vanes are generally three-dimensional. Nevertheless, the traction/compression component (along the axis of the vane) is predominant therein. Mechanical strength characteristics are orthotropic, and for a 3D woven preform, its traction strength can be five times greater than its compression strength. Unfortunately, conventional embedding by bolting does not take this feature into account since it does not enable guide vanes to be loaded differently in traction and in compression.
There therefore exists a need for such guide vanes to be fastened in different manner on the hub and the outer shroud of the intermediate casing, making it possible to both conserve the integrity of the composite part and also to optimize the transition of forces while taking account of the orthotropic characteristics of composite material so as to ensure that it is stressed in its preferred direction.